Devices that perform one or more tests on U.S. currency to determine their genuineness before dispensing change, commonly referred to as bill readers, are well known in the art. Typically in such devices, the paper currency being validated is moved over a path along which various optical, magnetic or edge-sensing tests are performed. 0n failing any of these tests, the bill is moved along the path in a reverse direction to be returned to the user, and no credit or change is given.
Such bill changers have become a target for vandals and robbers because they are subject to being tripped by stringing. Stringing occurs when a pull-back tape or string is attached to the trailing edge of a bill which is inserted into the bill acceptor portion of the bill changer for validation of the bill. Upon receiving change, the bill can be pulled back and reinserted again and again. Eventually, all of the change is emptied from the bill changer. Although certain methods and devices have been proposed to overcome this problem, such as the control circuit for a bill and coin changer disclosed in U.S. Pat. No. 4,503,963 to Steiner, these other devices are complicated and not easily adaptable to existing machines already in use.